412 PROCEEDINGS OP THE AMERICAN ACADEMY. 



+ 

 perimeuts, was as follows : The reaction. 2 H + 2 = H 2 , does not 



occur in a single stage, but in the two following: H + = H, and 

 2 H = II 2 . The reaction whose slowness causes the polarization is the 

 second of these. In other words, the potential of a hydrogen electrode 

 depends on the concentration of raonatomic hydrogen, and this concen- 

 tration increases rapidly during cathodic polarization on account of the 

 slowness of the reaction by which it is removed. For example, at a 

 polarization potential of 0.7 volt the concentration of this substance at 

 the electrode would be 10 1 ' 2 times as great as it would be when in equilib- 

 rium with ordinary hydrogen. Nevertheless, both of these concentrations 

 might be absolutely very small, and probably are. The difference in 

 polarization with different cathodes would be explained by the different 

 catalytic action of the material at the electrode, the polarization being 

 less the greater the catalysis. This theory, although it has been sug- 

 gested by Tafel,* has otherwise had no place in the numerous discussions 

 which the phenomenon of over-voltage has occasioned. It possesses, 

 nevertheless, a good deal of plausibility. We may in fact cause the 

 electrolytic reaction to proceed in the very two stages which we have 

 written above. When hydrogen is deposited on a palladium cathode, 

 the hydrogen, forming without marked polarization, is absorbed by the 

 metal, where it has been shown by two independent methods t to exist in 

 the monatomic condition. If the hydrogen is then withdrawn from the 

 palladium, it appears in the form of ordinary hydrogen, H 2 . 



But we have more striking arguments in favor of this theory. Those 

 metals, notably platinum and palladium, in whose presence the electro- 

 lytic deposition of hydrogen and the reverse reaction, the electrolytic 

 solution of hydrogen, progress most readily, are the very ones which we 

 have every reason to believe catalyze the reaction 2H ^1 H 2 (of course, 

 if in one direction, in both). This reaction is doubtless a very slow one 

 under most conditions. Hydrogen at ordinary temperatures is a pretty 

 inert substance, but in the presence of palladium or platinum black it 

 readily reduces a large number of substances. So also these metals aid 

 the union of hydrogen with other elements, such as oxygen and the 

 halogens. Furthermore, in every known case where hydrogen is pro- 

 duced by a reaction, the reaction is catalyzed by these metals. We may 

 mention the action of metals on acids, the reduction of water by chromous 



* Zeit. phys. Chem., 34, 200 (1900). 



1 Hoitsema, Zeit. phys. Chem., 17, 1 (1895). Winkelmann, Ann. der Phys., (4), 

 6, 104 (1901). 



